The invention relates to a color display device comprising a means for generating electrons and a substrate provided with a phosphor pattern, a color filter layer extending between the phosphor pattern and the substrate.
The invention also relates to a method of manufacturing a display device, in which a color filter layer and a phosphor pattern are provided on a substrate.
Color display devices of the type mentioned in the opening paragraph are used, inter alia, in television receivers and computer monitors.
A color display device of the type mentioned in the opening paragraph is known. The known color display device comprises a cathode ray tube incorporating an electron gun and a display window, the inner surface of said display window being provided with a phosphor pattern. Said phosphor pattern has sub-patterns of phosphor regions luminescing in red, green and blue (hereinafter also referred to as, respectively, xe2x80x9credxe2x80x9d, xe2x80x9cgreenxe2x80x9d and xe2x80x9cbluexe2x80x9d phosphors) and it may further comprise a so-called black matrix. A black matrix layer is a black layer having apertures or a system of black stripes on the substrate and (partly) between the phosphor regions from which the phosphor pattern is built up, and said black matrix layer improves the contrast of the image displayed. The black matrix is provided with apertures accommodating colored layers (also referred to as color filter layers) on which a phosphor region of a corresponding color is deposited. The color filter layers may also extend over the black matrix. The color filter layer absorbs incident light of other wavelengths than the light emitted by the relevant phosphor. This leads to a reduction of the diffuse reflection of the incident light and improves the contrast of the image displayed. In addition, the color filter layer (for example a xe2x80x9credxe2x80x9d layer) may absorb a part of the radiation emitted by the xe2x80x9credxe2x80x9d phosphor, namely the part whose wavelengths are situated outside the red portion of the visible spectrum. This results in an improvement of the color point of the red phosphor. The known color display device comprises a color filter layer for each of the phosphors (red, green and blue). For clarity, it is noted that xe2x80x9credxe2x80x9d, xe2x80x9cbluexe2x80x9d and xe2x80x9cgreenxe2x80x9d color filter regions have a relatively high transmission for red, blue and green light, respectively. The color indication of the color filter layers relates to the transmission properties of the filters, not to their color.
However, the effectiveness of the color filters in the known color display device is insufficient. The better the absorption spectrum is attuned to the light emitted, the greater the effectiveness of the color filter and the better the image display.
It is an object of the invention to provide a color display device of the type mentioned in the opening paragraph, which enables a better image display to be achieved.
To achieve this, a color display device of the type mentioned in the opening paragraph is characterized in that a transparent intermediate layer extends between the color filter layer and the phosphor pattern.
The invention is based on the recognition that the color filter layer and the phosphor pattern are excited by means of relatively high-energy electrons (approximately 25 kVolts of kinetic energy). A part of the electrons pass through the phosphor pattern, however, their kinetic energy level generally undergoes a reduction. Electrons which are passed by the phosphor pattern and reach the color filter layer may adversely affect the quality of the color filter layer in the course of time, so that the materials to be used for the color filter layer are subject to limitations. The electrons cause an ageing phenomenon in the color filter layer. As a result of said ageing phenomenon, the absorption spectrum of the color filter layer is subject to change. This adversely affects the quality of the image displayed. The provision of an intermediate layer causes the electrons to be stopped, at least in part, by said intermediate layer, so that fewer electrons reach the color filter layer. The intermediate layer is chosen to be transparent so that the intermediate layer passes light emitted by the phosphor pattern. Preferably, the intermediate layer comprises inorganic materials. In comparison with organic materials, inorganic materials exhibit a better resistance to electron bombardment and, at the same layer thickness, the number of electrons that is stopped is far greater. In embodiments of the invention, the materials used for the color filter are not stable when they are exposed to an electron bombardment with electrons having a kinetic energy above 7.5 kVolts, said materials being mainly organic pigments, such as (codification in accordance with Color Index) PR190, PR123, PR149, PR178, PR202, PR206, PV29, PB16, PB27 and ZnPc, Red 4013TR (manufactured by Ciba-Geigy). Preferably, the intermediate layer has a layer thickness d (in nm) which exceeds 25(A/rho)(E0/Z0.5)n, wherein A is the molecular weight, rho is the density (in gr/cm3), Z is the atomic number and n is given by n=1.2/(1-0.29 log10Z) and E0 is 7.5 (kVolts). Intermediate layers of such a thickness stop almost all electrons. The layer thickness preferably does not exceed 2 times the thickness indicated above by means of the formula. Layers having a larger thickness do not offer more protection. In comparison with organic materials, inorganic materials generally have a higher Z number and a higher density rho and are hence preferred to organic materials.
As a result, the invention enables a much greater variety of color filters to be used, so that the absorption spectrum of the phosphor and the color filter can be better attuned to each other. The color filter layers used hitherto, such as layers based on iron oxide (red color filters), cobalt aluminate (blue color filters) and CoO.NiO.TiO2.ZnO (green color filters) are by no means perfect. When use is made of the known color filter layers, the increase of the so-called LCP value (Luminance Contrast Performance, defined by the white Luminance (Lw) divided by the diffuse Reflection Rxc2xddiff (LCP=Lw/Rxc2xddiff)) amounts to only 20% of the increase achieved with a comparable color display device without color filter layers. By replacing iron oxide with, for example, Red 4013TR or cobalt aluminate with, for example, PB27, the increase of the LCP-value amounts to approximately 28%.
Preferably, the specific mass of the material of the inorganic layer is above 3 g/cm3. The higher the specific mass, the better the electrons are stopped. If the specific mass exceeds 3.0 g/cm3, the intermediate layer exhibits a considerable stopping effect, even if it is less than 1 micrometer thick. Sub-micron layers (layers having a thickness below 1 micrometer) can be applied more readily and exhibit fewer disadvantages.
A method of the type mentioned in the second paragraph is characterized in accordance with the invention in that an intermediate layer is provided between the color filter layer and the phosphor pattern.
In addition to the above-mentioned advantages, the method has the advantage that a reduction of phosphor haze is achieved. Phosphor haze occurs if phosphor particles of a specific color (for example red) adhere to regions which are intended for phosphor particles of another color, for example blue. This is an undesirable phenomenon which causes color impurities and hence a reduction of the quality of the image displayed. The provision of an intermediate layer between the color filter regions and the phosphor pattern reduces phosphor haze and hence improves the quality of the image displayed.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.